Brush wear indicator



Aug. 4, 1970 l H. A. THOMPSON 3,523,288

BRUSH WEAR INDICATOR Filed March 20, 1968 2 Sheets-Sheet l Fig. I3

INVENTOR. Harris A. Thompson A fr0/ME Ys Aug. 4, 1970 H. A. THcMPsoN3,523,233

BRUSH wma INDICATOR Filed March 2o, 1968 2 sheets-sheet z Fm 9 Fig. /0

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33 ,6' 32 3o 34 35 B/ 3l 32 \l6 16/ Fig. Fly-f2 4 I INVENTOR. Harris A..771 ompson BYWM A TTOR/VEYS 4United States Patent O- i 3,523,288 BRUSHWEAR INDICATOR Harris A. Thompson, P.O. Box 916, Boulder, Colo. 80302Filed Mar. 20, 1968, Ser. No. 714,555 Int. Cl. G08b 21/00; H01h 1/36U.S. Cl. 340-267 13 Claims ABSTRACT OF THE DISCLOSURE A brush wearindicator for an electric motor including a switch means having oneportion contacting a motor brush. This switch means trips to initiate awarning device. When the brush is nearly worn out, the contactingportion drops over the edge of the brush and the switch is tripped. Aswitch may be used with one or with each brush and when used with both,either will trip to actuate the warning device. The warning device maybe a light adapted to flash whenever the switch is tripped.

This invention relates to Wear indicating devices, and more particularlyto brush wear indicators in electric motors.

A primary object of the invention is to provide a novel and improvedbrush wear indicator for an electric motor which will actuate a suitableWarning signal whenever the brush becomes suiiiciently worn as torequire replacement.

The present invention is especially useful in artificial respirationdevices, especially those which must be run continuously to assist apatient in breathing. In such cases, should the apparatus be stoppedbecause of a breakdown, the life of the patient may be in danger andevery reasonable precaution must be taken to prevent such an occurrence.In such devices, the motor is usually a 12-volt type, which is normallyoperated by an alternating current supplied by a regular 11G-volt A C.power source, with the voltage being reduced by a transformer. However,in case of an emergency, the motor may also be operated by a directcurrent power source such as a storage battery.

By keeping a storage battery available for emergency uses, a powersupply is assured, and the only possibility of a breakdown will be inthe mechanisms of the respirator itself. However, by the selection ofgood components, the respirator mechanisms can be designed to operatereliably over long periods of time, with the only wear occuring on thecommutator brushes of the electrical motor driving the blower.Ordinarily, this brush Wear can be predicted and after several thousandhours of operation, the motor will be inspected, serviced and thebrushes replaced even though they may be only partially worn.

Naturally, such a procedure does not allow for contingencies such as anextraordinary rate of brush wear. Also, where a motor will run forseveral thousand hours without attention, the individuals relying uponthe mot-or often lose track of time, with the result that the brusheswill be worn out and the motor will stop, often at a very inconvenienttime. Also, arcing will occur across a commutator before the motor stopsto cause serious damage to the motor.

3,523,288 Patented Aug. 4, 1970 There is a real and definite need for animproved brush Wear indicator in a situation such as that aboveindicated, where an electric motor must run continuously and where thebrush wear is the component which requires the most frequent periodicattention. The above invention was conceived and developed with such aneed in view, and comprises in essence, an auxiliary switch built intothe brush assembly which is adapted to remain in a normally closed, oropen, position until such time as the brush becomes worn. Thereupon, theswitch trips to open or close to initiate a. warning signal, preferablya ashing light, which will operate continuously thereafter until themotor brushes are replaced.

Accordingly, another object of the invention is to provide a novel andimproved brush wear indicator for an electric motor which will include aWarning device to warn users that the brushes are almost worn out andmust be replaced before they are completely worn out.

Another object of the invention is to provide a novel and improved brushwear indicator which will emit a continual warning signal when the brushis about to be Worn out, so that the brushes may be replaced at any timewithinseveral Weeks after the warning signal commences as at a time whenit is also convenient for a regular serviceman to check and service themotor and other mechanisms in the apparatus.

Another object of the invention is to provide a novel and improved brushwear indicator which is especially adapted to use an electrical flashingsystem, of the type where the hashing light will attract attention untilthe matter is taken care of.

Another object of the invention is to provide a novel and improved brushwear indicator which is especially adapted to be used in electricalmotors of the type having unitized brushes and brush carriers forcertain components of the indicator may be advantageously incorporatedinto the unitized brush carriers as a part of a factory-produced item.

Other objects of the invention are to provide a novel and improved brushwear indicator which is a simple, reliable, 10W-cost arrangement, and iseasily incorporated into equipment such as respirator devices or also,into similar equipment where it is important to keep the electricalmotors continuously running.

vWith the foregoing and other objects in view, my present inventioncomprises certain constructions, cornbinations and arrangements of partsand elements as hereinafter described, deiined in the appended claims,and illustrated in preferred embodiment by the accompanying drawing inwhich:

FIG. 1 is an isometric view of a respirator, of a type which canadvantageously use the invention, formed as an enclosed container,having controls and indicators on its upper face and a blower within itas shown by broken lines.

FIG. 2 is an isometric View of a blower, such as is used in therespirator shown at FIG. l, and showing the motor portion of the blowerwherein the brush wear indicators are incorporated according to thepresent invention.

FIG. 3 is a plan view of a portion of the motor, as from the indicatedarrow 3 at FIG. 2, but on an enlarged scale, to show the commutator andone brush within its holder bearing against the commutator.

FIG. 4 is a side elevational view of the portion of the motor shown atFIG. 3.

FIG. 5 is a longitudinal sectional view as taken from the indicated line5 5 at FIG. 3, but on an enlarge scale.

FIG. 6 is a sectional fragment of the brush and its holder similar to aportion of the showing at FIG. 5, but on a greatly enlarged scale andwith the brush being worn to the point where the improved brush wearindlcator becomes operative.

FIG. 7 is a portion of the circuit diagram of the electric motormodified to include a circuit for a brush wear indicator, having a lightwhich will burn steadily while the brushes are of an operative length,but which will flash when one or both of the brushes are nearly wornout. f

FIG. 8 is a circuit diagram similar to FIG. 7, but with an arrangementhaving a light which will flash fora few minutes when the motor starts,but thereafter will not burn while the motor is running and the brushesare of an operative length, but which will ash when one or both of thebrushes are nearly worn out.

FIG. 9 is a plan view similar to FIG. 3, but showing a modified type ofbrush holder.

FIG. 10 is a side elevational view of the construction shown at FIG. 9.

FIG. 11 is a longitudinal sectional view of the brush and its carriersimilar to FIG. 5, but showing yet another arrangement wherein theelectrical contact mechanism of the wear indicator is reversed from thearrangement shown at FIG. 5.

FIG. l2 is a circuit diagram similar to that of FIG. 7, but being modiedto use the indicator shown in FIG. 11, using a separate light for eachbrush which will not burn while the motor is running and the brushes areof an operative length, but which flash when the brush is nearly wornout.

FIG. 13 is yet another circuit diagram modified to provide a wearindicator is one brush only, to provide for a continuously burning pilotlight which goes out when the brush is nearly worn out, and to providefor a manual switch to test the pilot light.

Referring more particularly to the drawing, FIG. l illustrates a typicalrespirator R which is one example of an apparatus wherein the presentinvention may be incorporated. This respirator is formed as a box-likecase 15 having a power supply lead 16 extending into the case to drivevarious mechanisms therein, such mechanisms including a motorized airblower A and control components not shown. A switch 17 and suitablegages and control knobs 18 are located on the surface of the case 15 forturning the unit on and for adjustments during its operation. Also, ablower tube 19 extends from a side of the case for attachment torespiratory equipment, not shown.

In the present invention, a brush wear indicator is incorporated in theelectrical motor of the blower A and it includes a signal means whichwill actuate whenever one, or both, of the brushes become worn to thepoint where replacement is required, all as will be hereinafterdescribed in detail. Preferably, such a signal means is a light L whichis conveniently mounted upon the resiprator case 15 and which will lightup, go out or continually flash whenever it is actuated depending uponthe type of circuit used with the light. The purpose of the light,regardless of the manner in which it operates, is to call to theattention of the users, the need for brush replacement and a periodiccheck of the apparatus to prevent a breakdown at an inopportune time,and thereby assure a long period of continuous operation of theapparatus.

The air blower A, as illustrated at FIG. 2, includes a cylindrical shell20 wherein a centrifugal blowing mechanism is housed, with a suitabledischarge tube 21 extending from one side of the shell to connect withthe blower tube 19. lIt includes also, an electrical motor M outstandingfrom an end of the shell and in axial alignment therewith. The motorshaft 22 shown in dotted lines at FIG. 2, extends into the shell 20 tocarry the rotor of the blower.

The motor M is a conventional arrangement housed within a suitablecylindrical frame 23 aixed to the shell 20 as by a transition 24. Thisframe includes a spider 25 at its outward end to carry a bearing 26 forthe end of the shaft 22. The shaft carries the rotor windings 27 and acommutator 28. The eld windings 29 are mounted within the frame aboutthe rotor and two diametrically opposed brush carriers B are mountedupon the edges of the frame 24 so that the brushes 30 within them bearagainst the commutator 28. Power leads 16' extend to this motor throughthe frame to connect with the field windings 27 and the brushes 30 torender the unit operative.

The present invention is concerned with indicating the wear of thebrushes 30 as they bear against the commutator and the inventionincludes a normally closed, or normally open switch which will beopened, or closed, by the position of the brush within its carrier Bjust before the brush is worn out.

The carrier B must insulate the brush 30 from the motor frame and thus,the construction on various types of electric motors is quite similar.In the construction illustrated, the carrier is generally formed as adeep cup-like body 31 of a plastic or similar insulating material. Ashort metallic tube 32, closed at one end, approximately one-thirdlonger than the body and essentially rectangular in cross section, ismounted within the body to form a socket with the forward portionthereof extending beyond the body 31. The brush 30 slidably fits in thissocket and a spring 33 also lies within this socket behind the brush andis adapted to urge the brush outwardly. A pliable feeder wire 34 ismounted in the tube within the embrace of this spring 33, and is securedto the bottom of the tube 32 and to the back of the brush 30. This wire34 thus connects the brush with the tube to provide a better electricalcontact between the two and also, limits the outward movement of thebrush from the tube when it is worn out. To complete this assembly, alead wire 16l is attached to the outer face of the metallic tube as by aclip 35, tightly fitting into a small space between the body 31 and tube32 as best illustrated at FIG. 5.

This body 31 is mounted upon a shelf portion 32a on the rim of the motorbody 23 by a clamp 36 extending over the body 31 and being held inposition by mounting screws 37 turned into socketed studs 38 upstandingfrom the shelf 23a. So mounted, the axis of the body is radiallyoriented wit-h respect to the commutator axis with the mouth of thesocket formed by the tube 32 being comparatively close to the commutator28. Accordingly, the major portion of the brush 30 will slidably extendinto the tube 32 with its outward end 39 projecting therefrom to contactthe commutator, as clearly illustrated at FIG. 5. It follows that as thebrush is worn, the spring 33 urges it outwardly from the socket andagainst the commutator, and that when the brush is worn to the pointwhere it is considered worn out, the retaining wire 34 is stretched tautpreventing further movement of the brush. It must then be replaced orelse the motor will soon stop running. Before it stops, however, thearcing from t-he brush to the commutator can damage the commutator.

To modify this brush carrier B and the brush 30 within it, to indicatewhen the brush is nearly Worn out, an indi-r cator switch S for eachbrush is mounted upon the respective carriers B. In the constructionillustrated at FIGS. 2 through 6, the indicator switch S includes aresilient arm 40, of fiat spring brass, or the like, which lies upon thetop of the body 31 to extend longitudinally thereupon with the forwardextended end being cantilevered over the portion of the tube 32extending beyond the body 31. This arm 40 is held in position by theclamp 36, but with a flat insulator pad y41 between the arm 40 and clamp36 to completely insulate the arm from the frame of the motor. A lockstrap 42 may also be wrapped about the portion of the body 31 whichextends outwardly from the frame ledge 23a, to extend also about the arm40 and insulator pad 41 as shown.

The indicator switch S also includes a slide pin 43 carried in aninsulator button 44 which fits with a press fit in an opening in the topsurface of the extended end of the tube 32 directly underneath thecantilevered end of the arm 40. The button 44 has a central passagewaythrough it wherein the body of the pin 43 slidably fits to extendthrough it and bear against the brush 30. The top of the pin includes anenlarged head 45 which is contacted by the undersurface of the arm 40.The length of the pin is such as to hold the arm 40 in a lifted positionand biased against the pin when the bottom of the pin within theinsulated button 43 is bearing against the top of the brush within thetube 32, as best illustrated at FIG. 5.

Accordingly, in the arrangement, a continuous electrical circuit extendsthrough the normally closed switch S, lead 50 wheren the light L islocated and this lead 50 extending from the brush 30, to and through thepin 43, and to and through the arm 40. The outward end of this arm,opposite the pin contact end, is upturned as at 46, to provide aconnector clip for the attachment of a lead wire 47 as by a drop ofsolder 48. The lead 47 is part of a signal means circuit as hereinafterdescribed.

The switch S, thus described, is normally closed and the signal meanswill be actuated when the switch is tripped to open and interrupt thecircuit through it. Tripping may be effected by locating the insulatorbutton and pin at a position on the tube 32 where the rear inner edge ofthe brush will pass as it approaches the worn-out position so that thepin will drop away from the brush. However, because of the chance thatthe pin might accidentally contact the spring 33, it is preferred thatthe pin drop into a hole 49 drilled into the top of the brush 30 nearits inner, rearward end as illustrated at FIG. 6. The hole 49 is severaltimes as large as the pin and it is to be noted that when the pin isdropped into this hole 49, the undersurface of the head 45 abuttingagainst the button 44 limits the depth to which the pin may drop so thatit will not contact the bottom of the hole. It is also to be noted thatthe size of the hole 49 is such that some brush wear will occur beforethe pin moves from the edge of the hole, its position when it firstdrops into the hole, to the middle of the hole as illustrated. Thus, thecircuit may be open to permit the signal means to operate for severalweeks before the motor stops.

The location of the hole 49 is correlated with the length of therestraining wire 34 so that as brush wear causes the hole 49 to shift tothe position shown at FIG. 6, or shortly therebeyond, the restrainingwire wire become taut to prevent further brush movement and toeventually stop the motor.

One or both brush carriers B may be provided with switches S, andvarious signal circuits may be used in connection therewith as will nowbe described.

Where two indicator switches S are used, a circuit such as thatillustrated at FIG. 7 may be used. This circuit shows that portion ofthe motor circuit associated with the brushes. The two power leads 16 tothe motor connect to the field coils and extend to the brushes 30 aslead 16". Since each switch S is normally closed by contacting itsbrush, the opposing leads 47 form a shunt connecting with a lead 40wherein the signal light L is mounted, between the leads 47. Thus, asllong as both switches S are closed, the signal light will emit asteady, glowing light whenever the motor is running. The steady lightwill indicate that the apparatus is turned on and is functioningproperly.

Each end of the lead 50 also connects with a lead 451 in parallel withthe shunt lead 47 and the lead 51 connects to the power supply lead 16back of the brush 30. Each lead 51 includes a asher generator F, of anysuitable type such as, for example, a Tungsol unit, Type 616,manufactured by Tung Sol Electric Co. Accordingly, it follows thatwhenever either indicator switch S opens because the brush wear permitsthe pin 43 to drop into a hole 49, the flow through the shunts 47 andlead 50 is interrupted and the light L goes out. Then the current mustflow through a lead 51 and this flow will actuate the flasher generatorF and cause the light L to flash to indicate that it is necessary toreplace a brush.

Another circuit, which may be used where it is desirable to have thelight L burn only when flashing is desired, is illustrated at FIG. 8.The leads 47 from each switch S are connected to a resistor 52, theheating element of a thermal relay of the type which uses a bimetallicswitch 53, which is normally closed and opens responsive to heat fromresistor 52. One commercial unit having such components is an Amperite5CT3 manufactured by Amperite Co., Inc. This switch 53 is in a circuitlead 54 which includes a flasher generator F and the signal light L in aseries arrangement. The lead l54 is connected directly to the oppositepower leads 16 back of the brushes 30, so that whenever the switch 53 isclosed, the flasher F causes the light L to flash. In operation, withthe motor running and the indicator switches S properly contacting theirrespective brushes, the resistor 52 is heated to hold the switch 53open. The switch 53 closes whenever a switch S opens as when a brush isworn out or when the motor is turned off. The light L will flash for ashort period of time immediately after the motor starts and before theresistor 52 is heated, but such is not significant and actually isdesirable since it serves as an indication that the flasher isoperative.

FIGS. 9 and 10 illustrate a modified form of a brush carrier B'. In thisconstruction, the plastic body 31 has a pair of opposing lugs 55 and itis held in position by clips 56 mounted on a flat shelf 23a of the motorbody as by mounting screws 37. The arm 40 is mounted upon a pad 57 andsecured in place by pins 58 which extend through both the arm and pad.To supplement the pins, a lock strap 42 embraces the assembly about thepad as illus` trated.

FIG. 1l illustrates one manner in which the arc indicator switch S' maybe mounted upon the brush carrier B to function as a normally open,rather than a normally closed, switch to be closed only when the brushis worn. In this construction, the components forming the brush carrierB, the body 31, the tube 32, spring 33, conductor wire 34 and circuitconnecting clip 35 are the same as hereinbefore described. Likewise,certain components of the switch S and the components which hold it inposition are the same as hereinbefore described. An arm 40 of a slightlymodified construction, cantilevers from the body 31 to overlie theinward end of the tube 32 and be held in position upon the body by aU-strap 36 with an insulator pad 41 between the arm and strap 36.Likewise, a lock strap 42 may fasten the arm 40 to the body 31 and theouter end 46 of the arm may be connected to a lead wire 47, asillustrated.

The button 44 is not an insulator button, but a conductor button, ofmetal, and is soldered or otherwise fastened to the tube 32. The slidepin 43', on the other hand, is a simple, cylindrical member formed of aninsulating, nonconductor material. This pin does not carry a head, butinstead, a suitable contact button 60 is affixed to the underside of theend of the arm 40. The length of the pin 43 is such that when it iswithin the passage way of the button 44 and bears against the top of thebrush, the contact button 60 on the arm is lifted above the conductorbutton 44. However, when it drops into the socket 49', the contact 60rests upon the Ibutton 44 making contact to close the circuit. It is tobe noted that the socket 49', as illustrated in FIG. 11, can beslotformed instead of being a hole and also, it can be eliminated withthe pin 43' being adapted to drop over the rear edge of the brush sincethe insulated pin will not cause an undesirable circuit if it contacts aspring 33.

FIG. 12 is a circuit which may be used with the construction illustratedat FIG. 8. The lead 47' from each switch S is conected to an individuallight L, a flasher F and to the power input circuit 16 at the oppositeside of the motor, Therefore, whenever a normally open switch S isclosed, the circuit is closed and the light L will commence to flash. Itis to be noted that individual lights L and individual flashers F willbe required for each circuit lead, however, special lights can be usedhaving separate filaments.

PIG. 13 is a circuit which produces a burning light as long as thebrushes are in good condition and which goes out to warn the user of theapparatus when the brushes are worn. Also, this figure shows a circuitwhich uses a switch S on only one brush B". The use of the single switchS is desirable in an electric motor where the wear of one brush isalways faster than the wear of the other, a condition comon in manytypes of motors. Accordingly, the switch S is located at the fasterwearing brush B", and the lead 47 therefrom, connects with a extends,thence, to the power circuit lead 16l opposite the brush B".

The circuit is closed and the light burns whenever the motor inoperating and until the switch kS opens because of brush wear. Toascertain whether or not the light goes out for this reason or is merelyburnt out, a check circuit lead 65 having a normally open manual switch66, shunts the switch S. Closing switch 66 will cause the light to turnon if brush wear is the cause of its going out.

The invention has been described for a twobrush, single-phase unit;however, it could be easily applied to a three-brush, three-phase unitif desired. Moreover it is obvious that others skilled in the art canbuild and devise similar alternate and equivalent constructions whichare nevertheless within the spirit and scope of my invention. Hence, Idesire that my protection be limited not by the constructionsillustrated and described, but only by the proper scope of the appendedclaims.

I claim:

1. A brush wear indicator system for an electric motor having acommutator and a brush slidably contained within a socketed carrier withits leading edge extending therefrom to contact the commutator, saidbrush being resiliently based to be moved from the carrier and againstthe commutator as it wears, and comprising in combination therewith:

a circuit generally shunting the brush and commutator;

a switch means in said circuit adapted to be tripped from a normalposition and to thereby disrupt the circuit;

a signal means actuated responsive to a disruption of the circuit; and

a holding means associated with the switch means and with the brushholding the switch means in its normal position when the brush isnormally within its carrier, but shifting to trip the switch means whenthe brush has moved towards the commutator to a position where it isnearly worn out, whereby to disrupt the circuit and actuate the signalmeans.

2. The organization set forth in claim 1, wherein:

said signal means is a light in said circuit; and

in said circuit which opens upon tripping;

said signal means is a light in said circuit; and

a shunting lead having a flasher, shunting said nor mally closed switch,whereby, when the switch is closed, the light will burn steadilyresponsive to current flow through the said circuit, but when the switchopens, the light will flash responsive to an intermittent current flowthrough the said shunting lead about the switch.

3. In the organization set forth in claim 1, wherein:

said switch means includes a normally closed switch in said circuitwhich opens upon tripping;

a switch control in said circuit; and

a second powered circuit including said signal means and a controlswitch adapted to be actuated by said switch control, said switchcontrol being adapted to hold said control switch open whenever thefirst said normally closed switch is closed, 'but to close whenever therst said switch is opened.

4. In the organization set forth in claim 1, wherein said holding meansincludes:

a pin mounted upon said carrier to project normally into the socketportion thereof to bear against a side of the brush when the brush isnormally within the socket, but to drop behind a rearwardly disposededge of the brush surface when the brush is moved toward the comutatorand is nearly worn out.

5. In the organization set forth in claim 4, wherein the brush includesa hole in its side near its rearward end in alignment with the pin toconstitute said rearwardly disposed edge so that the pin will drop intothe hole.

6. In the organization set forth in claim 1, wherein said switch meansincludes a resilient arm mounted upon the carrier having an end biasedtowards the socket therein, and said holding means includes a pincarried in a passageway extending through a hole in the wall of thecarrier and into the socket, said pin having its inner end bearingagainst a side of the brush and its outward end contacting the arm tohold the arm away from the carrier when the brush is in a normalcondition, but to have its inner end drop over an edge of the brush whenthe brush is moved close to its worn out position to release the arm andpermit it to move towards the carrier.

7. In the apparatus set forth in claim 6, wherein the pin is a conductorand said circuit extends through the brush and pin, and said switchmeans in thus normally closed whenever the pin bears upon the brush, butis adapted to be opened whenever the pin drops away from the brush.

8. In the organization set forth in claim 6, wherein the pin is aninsulator and said switch means includes an electrical contact on thecarrier adjacent to the pin which is adapted to be conducted by the armwhenever the pin drops away from the brush.

9. An indicator control switch means for indicating the wear of thebrush of an electric motor of the type having a commutator and asocketed carrier wherein the brush is slidably mounted and isresiliently biased to be urged therefrom and against the motorcommutator, said switch means being held in a normal position when thebrush is normally within its socket, but tripped when the brush is movedtowards the commutator to a position indicative of its being nearly wornout, and including in combination therewith:

a resilient means mounted upon the side of the carrier and beingresiliently biased toward the side thereof;

a passageway through the carrier wall aligned with said resilient means;and

a pin slidably carried in this passageway with its outer end being heldby the resilient means and being urged inwardly by the resilient meansand with its inner end being held against a side of the brush when thebrush is in a normal condition, but with its inner end being adapted todrop from an edge of the brush when the brush approaches its worn outcondition to thereby release the resilient means and permit the same tomove towards the carrier.

10. In the organization set forth in claim 9, wherein a hole is formedin the face of the brush near its edge, and the pin is adapted to dropfrom the side of the brush and from an edge of the hole and thereinto.

11. In the organization set forth in claim 9, wherein:

the resilient means is a conductor; and

the pin is a conductor forming a continuous circuit between the brushand to the resilient means when the brush is in a normal condition, topermit the same to function as a normally closed switch adapted to beopened when the pin drops away from the References Cited brush. 12. Inthe organization set forth in claim 9, wherein: UNITED STATES PATENTSthe resilient means is a conductor; 2,613,258 10/1952 Azno 340-282 thepin is an insulator member to hold the switch 5 En@ t l 310 239 l-turrows e va means 111 a normal Y 013611 190511011, and 3,350,61710/1967 Firth 340 267 X a contact means is mounted on the carrieradapted to contact the resilient means and close the switch when ALVINH. WARING, Primary Examiner the Pin drops away from the brush' P. PALAN,Assistant Examiner 13. In the organization set forth in claim 7, wherein10 the resilient means is an arm attached to the carrier with U'S' C1'X'R' one end cantilevered from its point of attachment. 20G-164;310-239, 245; 340--282 UNITED STATES PATENT OFFICE CERTIFICATE 0FCORRECTION Patent No 3 523,288

Harris A. Thompson August 4, 1970 It is certified that error appears inthe above identified patent and that said Letters Patent are herebycorrected as shown below:

Column 7y line 6l, "said signal means is a light in said circuit; and"should read said switch means includes a normally closed switch Signedand sealed this 23rd day of February 1971.

(SEAL) Attest:

WILLIAM E. SCHUYLER, JR.

Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer

